Oxygen gas has many uses. For example, it can be used for treatment of patients in the medical field, for various industrial processes, and for breathing in an environment in which oxygen is deficient. As a result of the variety of uses for oxygen gas, there is currently a substantial demand for such gas and also for a process by which it can be produced economically, efficiently and safely. Preferably such a process can be carried out in relatively large units and also in relatively small units, e.g. portable units.
One process that is presently used to produce oxygen gas is electrolysis of water. There are, however, several problems associated with electrolysis that make it unattractive. For example, electrolysis requires a large consumption of electrical energy and the oxygen gas produced can contain small amounts of hydrogen gas which must be removed before the oxygen can be used. Additionally the concomitant production of hydrogen along with oxygen during electrolysis presents serious safety hazards.
In addition to electrolysis of water, processes are available in the art for producing pure oxygen gas by separating oxygen from a gaseous mixture such as air.
The most widely used oxygen separation process is cryogenic liquifaction and distillation of air. Such cryogenic processes have several drawbacks, however; they are energy intensive with overall efficiencies of less than about 35-40 percent and they must be run in plants whose capacities exceed about 100 tons per day to take advantage of economy of scale. Because cryogenic units must be quite large to be economically feasible, smaller and/or portable units based on this technology are not available. Therefore, when a cryogenic process is used, the resulting oxygen usually must be shipped from a large central production facility to the end user. In this case the product oxygen is often transported as a liquid in expensive vehicles equipped with cryogenic dewars. The cost of the cryogenic process is further increased since the transport and storage of liquid oxygen is hazardous and thus, special precautions must be taken.
In addition to the above described cryogenic processes, oxygen can be separated from air by means of known electrochemical processes which are based either on a two-electron reduction of oxygen or a four-electron reduction. For example, U.S. Pat. No. 3,888,749 to Chong, U.S. Pat. No. 4,061,554 to Chillier-Duchatel et al and U.S. Pat. No. 4,300,987 to Tseung et al, disclose electrochemical processes for separating oxygen from air by means of a two-electron transfer. U.S. Pat. No. 3,410,783 to Tomter discloses electrochemical processes for separating oxygen from air by means of either two- or four-electron transfers.
Since the electrical current which must be passed through an electrolyte in an electrolytic cell for separating a given amount of oxygen from a gaseous mixture is directly proportional to the number of electrons (n) that reduces each molecule of oxygen, a four-electron process requires twice the amount of current that is required by a two-electron process. For perfectly reversible (ideal) electrochemical cells the voltage is fixed by the thermodynamic relationship: EQU .DELTA.G=-nFE (1)
where .DELTA.G is the free energy change, n is the number of electrons transferred per mole of material passing through the cell, F is the Farady (1F=96,490 Coulombs) and E is the reversible, equilibrium cell voltage. For the separation of oxygen from air .DELTA.G is fixed and is independent of the method of separation. Since .DELTA.G is fixed in the ideal case, the energy efficiency is 100% regardless of the value of n because the voltage varies to compensate for n. Consider, for example, two ideal cells, A and B, with n equal to 4 and 2 respectively, where both cells operate with a .DELTA.G of 9.6 kilojoules/mole (kj/mole). The total amount of power required to electrolyze 1 millimole/second (mmol/s) of material in cells A and B is listed below in Table I.
TABLE I ______________________________________ Power-Watts Cell n Voltage Amps(I) (P = E .times. I) ______________________________________ A 4 0.025 384 9.6 B 2 0.05 192 9.6 ______________________________________
Although, as is shown above for ideal cells, while cell A must pass 384 amps and cell B must pass 192 amps to electrolyze 1 mmol of material per second, the total power required is the same for both cells.
Any real situation is somewhat different however because of unavoidable cell resistances and irreversibilities which prevent 100% efficiency. Thus, as is described below in greater detail, when oxygen is separated from air in an actual (non-ideal) electrochemical cell, the cell with a lower n value will be more efficient.
For example, in the non-ideal cell the total power is defined by equation (2) below: EQU P=E(faradaic).times.I+I.sup.2 R(ohmic) (2)
Since the four-electron process (n=4) must pass 2 times as much current as the two-electron process (n=2) to produce a given amount of product in a given amount of time, the ohmic term for the four-electron process will be four times as large as for the two-electron process.
Thus, when equation (2) is applied to the example set forth above and an actual cell resistance of 0.001 ohms is assumed, the power requirement to produce 1 mmol/sec of material in cell A is 157 j/mmole while the power requirement is only 46.5 j/mmole in cell B.
In the foregoing discussion it was assumed that equal amounts of product were produced during equal time periods in cells with the same resistance. In practice, it is possible to lower the cell resistance by increasing the area of the electrodes. For example, by allowing cell A to have four times the electrode area as cell B, and assuming the resistance of cell A is consequently lowered four-fold, the energy requirements for the two cells (cells A and B) will be the same. Thus, under conditions of equal energy requirements the cell with a lower n value has an advantage in cell size. Since the cost of electrochemical cells scales roughly with electrode surface area the advantage of providing a cell using a relatively lower n can be substantial.
A person skilled in the art can design a cell in such a way that electrode area, current density, voltage, product output and rate are optimized to suit a particular need. In general the cell with smaller n value will have an advantage in one or more of these parameters.